1. Field of the Invention
This invention relates to an optical wavelength/frequency detection apparatus for detecting a wavelength or a frequency of a laser source.
2. Description of the Preferred Art
In recent years, it has been and is required in the fields of measurement and communication which make use of light to measure and detect a wavelength or a frequency of a laser source with a high degree of accuracy with a high resolution. For example, in an optical FDM transmission system, it is necessary to control the optical frequencies of a plurality of laser sources on the transmission side with a high degree of accuracy to arrange them on an optical frequency axis. However, the optical frequency of a laser source is subject to a variation from an external environment or aging. Therefore, the optical frequencies of the individual laser sources must be detected successively so that they may not be displaced from the values of them set at an initial stage. Accordingly, a wavelength (frequency) detection apparatus is required which can detect a wavelength (frequency) with a high degree of accuracy with a high resolution and is small in size and high in reliability.
FIG. 21 is a block diagram showing an optical wavelength/frequency detection apparatus which employs a Michelson interferometer. Referring to FIG. 21, the Michelson interferometer is denoted at 60 and includes a half mirror 53, a fixed mirror 64 and a movable mirror 65.
The half mirror 63 splits and combines light for measurement, that is, measurement light, and reference laser light. The fixed mirror 64 is fixed at a predetermined location in the Michelson interferometer 60. The movable mirror 65 is movable to vary the difference between the optical path lengths.
The optical wavelength/frequency detection apparatus includes, in addition to the Michelson interferometer 60, a fiber light inputting element 61 for inputting measurement light, and a collimator 62 for collimating incident light into parallel light.
The optical wavelength/frequency detection apparatus further includes an optical detector 66 for counting the number of interference pulses of measurement light, an amplifier (Amp) 67 for amplifying a signal from the optical detector 66, an analog to digital (A/D) converter 68 for converting an analog signal from the amplifier 67 into a digital signal, a fast Fourier transform (FFT) unit 69, and a spectrum display unit 70 for displaying a spectrum of light.
The optical wavelength/frequency detection apparatus further includes a He-Ne laser 71 for outputting a reference laser beam of a known wavelength, an optical detector 72 for counting the number of interference pulses of the reference laser beam, and a correction section 73 for adjusting the interval of a sampling signal.
In the optical wavelength/frequency detection apparatus of the construction, measurement light inputted from the fiber light inputting element 61 is introduced by way of the collimator 62 to the half mirror 63, by which it is split into two beams. The thus split two beams of measurement light are reflected by the fixed mirror 64 and the movable mirror 65, respectively, and then combined with each other by the half mirror 63. The optical path length difference then is produced by moving the movable mirror 65.
Interference pulses are thus produced from the two split beams of measurement light due to the optical path length difference and counted by the optical detector 66. The output of the optical detector 66 is amplified by the amplifier 67 and converted from an analog signal into a digital signal by the analog to digital converter 68.
Meanwhile, the He-Ne laser 71 outputs a reference laser beam of a known wavelength. The reference laser beam passes similar optical paths to those of the measurement light to produce interference pulses similarly. The interference pulses of the reference laser beam are counted by the optical detector 72. The output of the optical detector 72 is supplied as sampling interval information to the analog to digital converter 68 by way of the correction section 73.
The output of the analog to digital converter 68 is inputted to the fast Fourier transform unit 69, in which it is processed by required calculation processing. The output of the fast Fourier transform unit 69 is supplied to the spectrum display apparatus 70, on which a corresponding spectrum of light is displayed. The operator thus reads a wavelength or a frequency of the measurement light from the display.
The optical wavelength/frequency detection apparatus, however, has the following subjects to be solved for practical use since the optical system is constituted from parts of the bulk type. In particular, the optical wavelength/frequency detection apparatus has the subjects to be solved that the apparatus is large in size, that alignment of optical axes is difficult and that optical axes are displaced from each other by vibrations OF some other cause. Also it is a subject of the optical wavelength/frequency detection apparatus that it is low in long term reliability since a motor or a like element is used in order to feed the movable mirror.